1. Technical Field
The present invention relates to a D/A conversion circuit, an oscillator, an electronic apparatus, and a moving object.
2. Related Art
JP-A-2001-111428 discloses a digital to analog (DAC) converter in which a switch for selecting a voltage dividing resistor of a D/A converter, which has been hitherto constituted by an N-channel type MOS transistor and a P-channel type MOS transistor, is constituted by only an N-channel type MOS transistor. According to such a D/A converter, since a P-channel type MOS transistor formed in an N-well is not used, a voltage dividing resistor may not be disposed in the vicinity of the N-well, and thus it is possible to solve a problem of the related art that a distortion rate deteriorates at the time of performing analog conversion due to a refractive index of light being influenced by a slight level difference between the height of the N-well region and the height of a P-type substrate except for the region and the finish widths of polysilicon, constituting a resistor, being different from each other in the vicinity of the N-well and other locations.
Incidentally, when an increase in bits of a D/A converter is advanced in order to improve a resolving power of the converter, the area of a resistive element constituting a voltage dividing resistor hardly changes, but the number of MOS transistors functioning as switches is increased, and thus the whole area is considerably increased. On the other hand, in the layout design of the D/A converter, when there is an attempt to make an area small by disposing the MOS transistor in proximity to the resistive element, it is not possible to sufficiently secure a distance between the resistive element and a gate electrode of the MOS transistor. Then, in a process of manufacturing a semiconductor integrated circuit, a resistive element is generally manufactured after a process of creating a gate electrode. For this reason, when the resistive element and the gate electrode are disposed so as to be excessively close to each other, a level difference is generated in the base of the resistive element due to the gate electrode. Accordingly, a level difference is also generated in a resist in the subsequent photolithography process, and defocusing occurs by exposure at an end in a region in which the resistive element is manufactured, due to a difference between the height of the resist and the original height of the resist, which results in an increase in an exposed region (FIG. 17A)). Then, in the subsequent etching process, the resist in a region in which the resistive element is originally desired to be manufactured is excessively removed, and the size of the resistive element is determined along the edge of the resist, and thus the width of the resistive element becomes smaller than its original width (FIG. 17B)).
Accordingly, a portion of the resistive element which is close to the gate electrode of the MOS transistor becomes thinner, but a portion thereof which is not close to the gate electrode of the MOS transistor does not become thinner, which results in a difference in a resistance value of a voltage dividing resistor constituting the resistive element, and thus there is a problem in that it is not possible to realize a highly accurate D/A converter.